1. Field of the Invention
The present invention relates generally to surface texture measuring instruments, and more particularly to surface texture measuring instruments for measuring surface texture of a workpiece utilizing evanescent light (near-field light).
2. Description of Related Art
An optical microscope has been known as an example of a surface texture measuring instruments for measuring surface texture of a workpiece utilizing evanescent light (near-field light), which is disclosed in Document 1 (JP-A-9-243649).
The optical microscope includes an optical waveguide probe having, at a tip end thereof, an extremely small opening with the diameter smaller than the wavelength of the light, an evanescent light generator that generates evanescent light on a surface of a sample, and an evanescent light detector that condenses and detects scattered light of the evanescent light, which is scattered at the tip end of the optical waveguide probe when the optical waveguide probe is moved close to the vicinity of the surface of the sample.
When the optical waveguide probe is moved close to the vicinity of the surface of the sample, the evanescent light generated on the surface of the sample is scattered at the tip end of the optical waveguide probe, and the scattered light is detected by the evanescent light detector. While scanning the sample with the probe along the surface of the sample, if a distance between the sample and the probe is controlled to keep the intensity of the scattered light constant, i.e., if the probe is vertically moved against the sample, the tip end of the probe can hold a fixed distance from the surface of the sample, thereby obtaining the surface texture of the sample by detecting a vertically moving amount of the prove.
Heretofore, in the optical microscope utilizing the near-field light, in order to obtain distance information, it is necessary to obtain in advance a relationship between a distance from the probe at the tip end of the detector to a workpiece (sample) and detection information, more particularly, the intensity of the scattered light of the evanescent light, which is variable according to the distance. However, the above-described relationship is strongly material-dependent regarding the workpiece. Owing to this, it is extremely difficult to obtain the relationship in advance in view of the material-dependency.
To overcome such difficulty, it is conceivable to constantly perform calibration with another measuring instrument such as a laser interferometer.
However, since mechanical and optical elements of the measuring instrument must be disposed in positions different from the position of the near-field scanning optical microscope, there are generated uncertainty of calibration in terms of time and space. Especially, the uncertainty in terms of space may be a serious bottleneck for the near-field scanning optical microscope that performs nanometer-length measurement on the workpiece.
In other words, the bottleneck is the difference in measurement properties between the length measurement position of the measuring instrument and the length measurement position of the near-field scanning optical microscope. Or, if the measuring instrument is moved to the length measurement position of the near-field scanning optical microscope in order to solve this bottleneck, the movement axis generated in the movement will cause further uncertainty, thus resulting in submicrometer-level uncertainty.